In her comments, Secretary Deborah Lee James stressed the importance of the US-led air campaign, but admitted that airstrikes need to be backed by ground forces.

"Air power is extremely important. It can do a lot but it can't do everything," James said, just two days after Secretary of Defense Ash Carter supported President Obama's "willingness to do more" in terms of US troops on Syrian ground.

"Ultimately it cannot occupy territory and very importantly it cannot govern territory," James told reporters at the Dubai Airshow. "This is where we need to have boots on the ground. We do need to have ground forces in this campaign."